1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal panel of a matrix type having picture element electrodes arranged in rows and columns and, more specifically, to a method of manufacturing a liquid crystal panel having picture element electrodes arranged in rows and columns, and a black matrix or a black mask for shading blank spaces formed between the picture element electrodes.
2. Description of the Prior Art
A liquid crystal panel having transparent electrodes is provided with a black mask formed on a substrate to prevent the degradation of the contrast of an image displayed thereon by light transmitted through blank spaces between the transparent electrodes. Methods of forming a black mask are classified roughly into three categories. A method of a first category forms a black mask by patterning a metal thin film, such as a chromium thin film, by a photolithographic etching process. A method of a second category forms a black mask by spreading a photoresist material containing particles of black pigment dispensed therein in a photoresist film over the surface of a substrate and patterning the photoresist film by a photographic process. A method of a third category forms a black mask by printing black ink over the surface of a substrate in a film by an offset printing process and subjecting the film of the black ink to a heating process. The method of the third category is disclosed in, for example, Japanese Patent Laid-open (Kokai) No. She 63-180933.
The method of the first category comprises a film forming process for forming a metal thin film, such as a chromium thin film, over the surface of a substrate by sputtering or vacuum evaporation, a photoresist film forming process for forming a photoresist film over the metal thin film, a photographic process for forming a photoresist mask by patterning the photoresist film, an etching process for etching the metal thin film in the pattern of a black mask, and a coating process for forming an overcoating layer or an insulating layer over the black mask of the metal thin film to insulate the black mask from the transparent electrodes to be formed over the metal thin film. Since the number of processes of the method of the first category is relatively large, the manufacturing cost of the black mask is relatively high.
The method of the second category also needs many processes. For example, a photoresist material containing particles of pigment dispersed therein is spread over the surface of a glass substrate by a spin coating process or a printing process to form a photoresist film. After prebaking the photoresist film, an oxygen-shielding material, such as PVA, is applied to the photoresist film to prevent free radicals produced in the photoresist film when the photoresist film is exposed to light from being deactivated through reaction between the free radicals and oxygen, the photoresist film is prebaked again, the photoresist film is exposed, using a photomask, the exposed photoresist film is processed by a photographic process, and then the photoresist film is subjected to a postbaking process to finish a black mask. Then, the black mask is coated with a overcoating layer.
Then, a transparent conductive film is formed over the coating layer, a photoresist film is formed over the transparent conductive layer, and then the photoresist film is patterned in a positive photoresist mask, using a photomask. The photomask must be correctly registered relative to the black mask. If the photomask is registered incorrectly, gaps will be formed between the black mask and transparent electrodes formed by patterning the transparent conductive film. Light transmitted through the gaps deteriorates the contrast of an image displayed on the liquid crystal panel having such incorrectly patterned transparent electrodes. A technique proposed previously to prevent forming such gaps forms the black mask by a relatively thick lines so that the lines forming the black mask over lap the edges of the transparent electrodes. Although the black mask having such relatively thick lines facilitates registering the photomask relative to the black mask, the numerical aperture of the picture element electrodes is reduced.
The method of the third category forms a black mask simply by printing black ink by offset printing and hence the number of processes of the method of the third category is relatively small. However, the printing accuracy of offset printing is lower than the accuracy of patterning the transparent conductive film. Therefore, the method of the third category, similarly to the method of the second category is obliged to sacrifice the numerical aperture of the picture element electrodes to compensate errors in registering the printed black mask relative to the pattern of the transparent electrodes.